libbinder_ndk: remove NullableStrongBinder

Nullable types do not exist in Java. For most Java types, the type is
split into a nullable and non-nullable variant. This is because C++
types are more usually non-nullable, but everything in Java is
non-nullable. This does mean that some Java interfaces may have to have
'@Nullable' added to them in order to function as expected w/ the NDK.
It also means that some transactions will be allowed in Java which are
not allowed in C++. However, in Java, if a null is ignored, it will
just result in a NullPointerException and be delivered to the other
side. C++ does not have this same capacity (in Android), and so
instead, we distinguish nullability in the type system.

The basic layout is:
NDK layer - all objects are nullable if they are nullable in Java
NDK C++ wrapper - functions are provided for nullable and non-nullable
    variants
AIDL - @Nullable selects nullable wrapper if desired, but the default
    is to return an error to the remote process (like Java would throw
    NullPointerException).

Bug: 111445392
Test: android.binder.cts
Change-Id: I615420f735a45ef7869b9ee7e86ce21b844d84a2